1. Field of the Invention
This invention relates to a sinterable base composition for preparing an intermediate layer between dental porcelain and a high-melting dental metal alloy, especially a nonprecious metal alloy, as well as to a method of preparing such an intermediate layer.
2. Discussion of Prior Art
Sinterable base compositions are used in denistry in order to improve the adhesion between a metal coating, usually of a nonprecious metal alloy, and a dental porcelain which is fused on afterwards. In the making of dentures, especially of crowns and bridgework, dental porcelain is applied to a metal substructure, a crown for example, which must be especially pretreated and cleaned, and it is fired on in layers at high temperatures, so as to achieve an aesthetically pleasing denture.
It has been found, however, that the adherence of the dental porcelain to the metal is in many cases unsatisfactory and depends to a high degree on the ability of the dental technician. This is the case especially when the nonprecious metal alloy is one of the alloys now on the market which are made on a basis of nickel, chromium and/or cobalt, which may also contain other additives such as molybdenum, aluminum, silicon, manganese, iron etc. Nonprecious metal alloys are today increasingly used for reasons of cost, and they are sufficiently described in the literature.
In order to improve the adhesion between metal and dental porcelain, a variety of binding agents have already been proposed. According to German Offenlegungsschrift No. 25 25 274, gold powder, dental porcelain, zirconium dioxide and liquid fluxes are used in certain proportions as binding agents between dental porcelain and metal. The liquid flux also contains vehicle substances, such as glycerol or alcohol. It can consist of boron oxide or the salts thereof, such as sodium borate, or the oxides of the light elements, such as lithium oxide, sodium oxide etc. During firing, the flux, together with the dental porcelain and the oxides, forms on the alloy a tough intermediate layer of oxides of low solubility, to which the dental porcelain adheres strongly during the subsequent firing.
A mixture of boron and aluminum together with an organic vehicle component is known as an adhesive composition from German Offenlegungsschrift No. 26 12 554. According to this disclosure, an uniform layer of boron and aluminum dispersed in petroleum jelly, for example, is applied to a cleaned metal substrate, and then coated metal is rapidly raised to a high temperature and the binding agent is fired on. The coated surface is cleansed after cooling, and the dental porcelain can be fired on in the conventional manner.
German Offenlegungsschrift No. 26 26 092 discloses a binding agent containing an aluminum powder, a glass powder and an organic vehicle. The vehicle component produces a reducing atmosphere during the sintering. The glass is a mixture of noncrystalline oxides which include SiO.sub.2, Al.sub.2 O.sub.3, Na.sub.2 O, CaO, MgO, K.sub.2 O, SnO.sub.2, Li.sub.2 O, B.sub.2 O.sub.3, Fe.sub.2 O.sub.3 and BaO.
Also pertaining to the state of the art German Offenlegungsschrift No. 26 32 871 are methods in which a mixture of Ni-Cr-Al and ceramic (Al.sub.2 O.sub.2, ZrO.sub.2, TiO.sub.2) is deposited from a vapor on the metal substrate.
Also the production of an intermediate layer consisting of gold, porcelain and ZrO.sub.2 and a flux is known from U.S. Pat. No. 4,181,757.
However, none of the above-described adhesives or binding agents is as yet satisfactory with regard to the improvement of adhesion between the metal substrate and the dental porcelain. Some of the processes are complex and expensive, and some of the binding agents improve only the chemical bond between metal and porcelain.